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Media Statement

Seasonal Influenza Vaccine and Risk of 2009 H1N1 Influenza

Background
Over the past week, there have been several media reports describing unpublished findings from seasonal influenza vaccine studies conducted in Canada. The findings from these studies suggest that getting the 2008-09 seasonal influenza vaccine (given last influenza season) increases a person’s risk of getting 2009 H1N1 influenza. In the studies done in Canada, the increase in risk among vaccinated persons is approximately double the risk for unvaccinated persons. These studies could not determine which seasonal influenza vaccine had been given. This research has not been published in the medical literature or presented at a public scientific meeting. The results of these studies have been presented by Canadian investigators to CDC scientists, but there has not yet been an opportunity to review the studies in detail.

CDC Response
Preliminary results of studies conducted in the United States using methods similar to the Canadian studies suggest that receiving a seasonal influenza vaccine does not increase the risk of becoming ill with 2009 H1N1 influenza. In addition, no other country has reported this finding. Only one study has been published on this issue, which reported data collected in Australia. The Australian study did not find any association between receipt of seasonal influenza vaccine and risk of getting 2009 H1N1 influenza. Australian researchers recently published these findings: Kelly H, Grant K. Interim analysis of pandemic influenza (H1N1) 2009 in Australia: surveillance trends, age of infection and effectiveness of seasonal vaccination. Eurosurveillance Volume 14, Issue 31, 06 August 2009. (http://www.eurosurveillance.org/ViewArticle.aspx?ArticleId=19288). In addition, seasonal influenza vaccination leading to increased susceptibility to other influenza viruses has never been reported before.

CDC is working with scientists in Canada, the World Health Organization (WHO) and other countries to further investigate the findings from Canada and other countries. Studies in the United States also are continuing. Thus far, why findings from studies in Canada have been different from results of studies conducted in other countries is unknown.The influenza virus strains used to make the Canadian seasonal influenza vaccine were the same as used in the United States and many other countries. However, some seasonal influenza vaccines used in Canada are not available in the United States and vice versa. The Canadian studies reportedly cannot determine which vaccines study participants received.

CDC continues to recommend seasonal influenza vaccination for the 2009-10 influenza season. Currently the vast majority of influenza being reported to CDC is 2009 influenza A (H1N1). However, influenza is very unpredictable and seasonal influenza viruses might circulate at any point in the season. CDC does not recommend halting or deferring seasonal influenza immunization efforts.

The recommendations for who should get seasonal influenza vaccine have not changed. CDC recommends seasonal influenza vaccination for anyone who wants to reduce their chances of getting seasonal flu. Yearly vaccination is particularly important for certain groups of people, including those who are at high risk of having serious seasonal influenza -related complications or people who live with or care for those at high risk for serious seasonal influenza -related complications, including:

Children aged 6 months up to their 19th birthday

Pregnant women

People 50 years of age and older

People of any age with certain chronic medical conditions

People who live in nursing homes and other long-term care facilities

People who live with or care for those at high risk for complications from flu, including:

Health care workers

Household contacts of persons at high risk for complications from the flu

Household contacts and out of home caregivers of children less than 6 months of age (these children are too young to be vaccinated)